Re: [Qemu-devel] [PATCH v2 14/18] xen: add implementations of xen-block connect and disconnect functions...

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Anthony PERARD [mailto:anthony.perard@citrix.com]
> Sent: 07 December 2018 18:21
> To: Paul Durrant <Paul.Durrant@citrix.com>
> Cc: qemu-devel@nongnu.org; qemu-block@nongnu.org; xen-
> devel@lists.xenproject.org; Stefano Stabellini <sstabellini@kernel.org>;
> Kevin Wolf <kwolf@redhat.com>; Max Reitz <mreitz@redhat.com>
> Subject: Re: [PATCH v2 14/18] xen: add implementations of xen-block
> connect and disconnect functions...
> 
> On Thu, Dec 06, 2018 at 03:08:40PM +0000, Paul Durrant wrote:
> > ...and wire in the dataplane.
> >
> > This patch adds the remaining code to make the xen-block XenDevice
> > functional. The parameters that a block frontend expects to find are
> > populated in the backend xenstore area, and the 'ring-ref' and
> > 'event-channel' values specified in the frontend xenstore area are
> > mapped/bound and used to set up the dataplane.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> 
> With this patch, we should be able to have QEMU instantiate a new
> backend for a guest, right ? (via command line or QMP)
> 
> I've tried, and that doesn't work, the xenstore path for the frontend is
> wrong. In the qemu trace, I have:
>     xs_node_create /local/domain/0/backend/xen-disk/23/268572709
> Which is probably fine, even if not described in xenstore-paths.markdown.
>     xs_node_create /local/domain/23/device/xen-disk/268572709
> Which is not, instead of "xen-disk", we should have "vbd".
> 
> I know that this is fixed in "xen: automatically create
> XenBlockDevice-s", but at least the "vbd" type couldn't be added in this
> patch, or maybe a previous one.


Yes, I guess I should move the name overrides into an earlier patch.

> 
> 
> Another issue seems to be error handling. I've done a very simple test,
> I've added '-device xen-disk,vdev=d536p37,id=mydisk' to the command
> line (which is obvious wrong), and QEMU abort with:
>     qemu-system-i386: hw/block/xen-block.c:174: xen_block_realize:
> Assertion `conf->blk' failed.
> But I've pointed out the error in the code below.
> 
> 
> And just for fun, adding then removing a xen-disk via QMP. Adding works
> fine (once I've fixed the frontend name). I've run the following with
> ./scripts/qmp/qmp-shell:
> blockdev-add driver=file  filename=/root/vm/disk/testing-disk.qcow2 node-
> name=emptyfile
> blockdev-add driver=qcow2 node-name=emptyqcow2 file=emptyfile
> device_add driver=xen-disk vdev=xvdn id=fromqmp drive=emptyqcow2
> 
> But, then, remove doesn't work, running "device_del id=fromqmp" doesn't
> do anything. I guess we can try to fix that later if you don't find
> what's missing.

Hmm, that's weird. I guess the name lookup must be failing somewhere.

> 
> > @@ -76,6 +151,7 @@ static void xen_block_realize(XenDevice *xendev,
> Error **errp)
> >      const char *type = object_get_typename(OBJECT(blockdev));
> >      XenBlockVdev *vdev = &blockdev->vdev;
> >      Error *local_err = NULL;
> > +    BlockConf *conf = &blockdev->conf;
> >
> >      if (vdev->type == XEN_BLOCK_VDEV_TYPE_INVALID) {
> >          error_setg(errp, "vdev property not set");
> > @@ -90,6 +166,59 @@ static void xen_block_realize(XenDevice *xendev,
> Error **errp)
> >              error_propagate(errp, local_err);
> 
> You probably want to add a return here, this is when
> `blockdev_class->realize' fails.
> 

Yes, I do.

> >          }
> >      }
> > +
> > +    /*
> > +     * The blkif protocol does not deal with removable media, so it
> must
> > +     * always be present, even for CDRom devices.
> > +     */
> > +    assert(conf->blk);
> 
> That assert should probably not be there, as a missing conf->blk isn't a
> programming error, but a user error, I think.
> 
> Actually, the issue is the missing return abrove, and the assert is
> probably fine.
> 

Yes, the assert is intentional and caught the programming error :-)

  Paul

> --
> Anthony PERARD